The present invention has for its object to provide a digital method of modifying an image to obtain an output image on the basis of an input image, each image being defined by a set of picture elements, denoted pixels hereinafter, each pixel containing at least a digital value defining its grey level among a set of possible values, said method comprising: a first stage in which a histogram is established which is representative of the distribution in a percentage figure of the grey level of the pixels of said input image, a second stage in which, on the basis of the distribution characteristic of said histogram a correspondence function is established between the input and output grey levels, a third stage in which the output image is formed on the basis of the pixels of the input image and of said correspondence function,
said correspondence function consisting of clipping the pixels having extreme grey levels and of linearly re-distributing the grey levels of the remaining pixels.
The present invention also has for its object to provide a digital system arranged for putting the method into effect.
This method applies to low-contrast images, more specifically infra-red images whose weak dynamic on the brightness scale is well known. The histogram of allow-contrast digitized image occupies only a small portion of the available grey levels.
In order to enhance the contrast, a known method is to linearly re-distribute, after clipping of the extreme levels, the input grey levels over all the available grey levels; however, this method has at least two disadvantages, as it has been found that in the first place in the case in which the input histogram is sufficiently large there is however, a gain in contrast which is not useful and often detrimental, in the second place for certain images in which there are small warm objects on a grey background, clipping the extreme pixels by a predetermined percentage has for its result that these small objects disappear, which is contrary to the envisaged object.
The present invention has for its object to obviate these disadvantages.